Scandium is an extremely valuable element as it is used as a high strength alloy by being added to aluminum or magnesium or as a stabilizer in the electrolyte of a fuel cell using zirconium.
Scandium has a particularly small ionic radius among the rare earth elements, and it has been thus known to be scarcely present in ordinary rare earth minerals but present in trace amounts in oxide ores of aluminum, tin, tungsten, zirconium, iron, nickel, and the like. However, scandium has not yet been used widely due to the small production quantity and high cost thereof.
In recent years, the high pressure acid leach (HPAL) process has been emerging as a practical method, in which nickel oxide ore is introduced into a pressure vessel along with sulfuric acid, and heated at a high temperature of about 240° C. to 260° C. to allow separation into a leachate containing nickel and a leach residue. In the HPAL process, for example, a neutralizing agent is added to the leachate obtained to separate and remove impurities, and then a sulfurizing agent such as hydrogen sulfide gas is added to the resulting leachate, allowing separation and recovery of nickel as a sulfide. The obtained nickel sulfide is subjected to a known nickel refinement process to be purified to a metal such as electric nickel or to a nickel salt compound such as nickel sulfate or nickel chloride.
In the HPAL process as described above, scandium contained in nickel oxide ore will be leached in a leachate along with nickel as described in Patent Document 1, but scandium is not precipitated by a series of treatments in which a neutralizing agent and a sulfurizing agent are added to the leachate but remains in the acidic solution after the sulfuration treatment and is separated from nickel.
However, scandium in the acidic solution is only contained at a dilute concentration of about several tens of mg/L, while impurities such as iron, aluminum, magnesium and the like are contained at higher concentrations. For this reason, the acidic solution containing scandium after sulfuration has been conventionally neutralized and disposed as wastewater sludge along with other impurities and has not been thus effectively used.
Accordingly, in order to enrich and refine scandium for effective utilization, for example, a method as disclosed in Patent Document 2 has been proposed. The method disclosed in Patent Document 2 is a method for recovering scandium which includes a leaching step of introducing nickel oxide ore containing scandium, aluminum, and chromium into a pressure vessel along with sulfuric acid and subjecting the nickel oxide ore to solid-liquid separation under high temperature and high pressure to obtain a leachate and a leach residue, a neutralization step of adding a neutralizing agent to the leachate to obtain a neutralized precipitate and a post-neutralization liquid, a sulfuration step of adding a sulfurizing agent to the post-neutralization liquid and separating it into nickel sulfide and a post-sulfuration liquid, an ion exchange step of adsorbing scandium to a chelating resin by bringing the post-sulfuration liquid into contact with the chelating resin to obtain a scandium eluate, a solvent extracting step of bringing the scandium eluate into contact with an extractant to obtain a backward extraction liquid, a scandium precipitating step of adding a neutralizing agent or oxalic acid to the backward extraction liquid to obtain a precipitate, and a roasting step of drying and roasting the precipitate to obtain scandium oxide.
According to such a method in which an ion exchange method and a solvent extraction method are combined, it is possible to efficiently recover scandium.
However, in this method, a scandium hydroxide or scandium oxalate precipitate is obtained by adding a neutralizing agent or oxalic acid to a solution containing scandium. Some or most of impurity metals such as aluminum and iron contained in the solution are also crystallized at the same time in the method for crystallizing scandium as a hydroxide. This makes selective separation of scandium difficult. Further, a hydroxide of scandium to be obtained takes a gel form, resulting in poor handling properties such as long filtering time.
Meanwhile, a reaction (hereinafter, simply referred to as the “oxalate-formation”) to obtain an oxalate salt of scandium by adding oxalic acid ((COOH)2) to the solution containing scandium has an advantage of better handling properties such as filterability.
In addition, Patent Document 3 discloses a method for producing high purity scandium triflate in which the pH of a low purity scandium compound-containing aqueous solution is adjusted to 0.5 to 4.0, scandium is then recovered as scandium oxalate by adding oxalic acid (salt) to the solution, the scandium oxalate is calcined to be converted into scandium oxide, the scandium oxide is dissolved in and reacted with trifluoromethanesulfonic acid to obtain a scandium triflate aqueous solution.
However, in the case of using the treatment method by oxalate-formation shown in Patent Document 2 and Patent Document 3, there are problems that the scandium-containing acidic solution of sulfuric acid contains a large amount of aluminum ions and ferrous (II) ions and the precipitation of aluminum oxalate and ferrous oxalate (II) also occurs at the same time. This is because the solubility of ferrous oxalate (II) in water is as low as 0.022 g/100 g.
In order to prevent the precipitation of ferrous oxalate (II), a method is used in which an oxidizing agent such as hydrogen peroxide is added to the solution so that the oxidation-reduction potential (ORP) of the solution is about 700 mV at the potential using the silver-silver chloride electrode as a reference electrode to oxidize ferrous (II) to ferric (III), and ferric oxalate (III) having high solubility in water is generated, thereby preventing precipitation of ferrous oxalate (II).
Such a method for generating ferric oxalate (III) can exert a sufficient effect when the iron ion concentration is lower than the scandium concentration. However, it is required to add a larger amount of oxidizing agent as the iron ion concentration increases. In addition, in order to oxidize ferrous (II) until the oxidation-reduction potential reaches about 700 mV, an oxidizing agent such as hydrogen peroxide or ozone which has a strong oxidizing power is required, and this increases costs and causes various problems such as durability of facilities and safety of handling. Meanwhile, when the addition of oxidizing agent is insufficient, precipitation of an oxalate occurs, resulting in a decreased grade of scandium and unstable operation.
Accordingly, in order to increase the solubility of each impurity, a method is conceivable in which the addition amount of oxalic acid is increased to be equal to or more than the equivalent amount required for the oxidation of scandium or the impurity metal concentration is decreased by diluting the starting liquid. However, it is not an advisable plan to perform such a treatment since the capacity of equipment required and the investment cost increase.
As described above, a method for efficiently recovering scandium from a solution containing a large amount of iron and aluminum ions has not been proposed, and it has been difficult to easily obtain high purity scandium which hardly contains such impurities.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2000-313928    Patent Document 2: PCT International Publication No. WO2014/181721    Patent Document 3: Japanese Unexamined Patent Application, Publication No. H09-248463    Patent Document 4: Japanese Unexamined Patent Application, Publication No. 2005-350766